The present invention relates to a high-speed recording apparatus in which a laser beam is modulated into plural independent modulated beams.
In a laser beam recording apparatus utilizing an acousto-optical modulator, a high-speed recording can be achieved by converting a single beam from a light source into plural independent beams by simultaneously applying ultrasonic waves of plural frequencies to the acousto-optical element and scanning a recording medium simultaneously with said plural beams modulated independently.
In a scanning system for single beam scanning by applying an ultrasonic wave of a single frequency to the acousto-optical element, the primary diffraction intensity is maximized when the incident beam angle to the acousto-optical element is so selected that the primary diffraction light satifies the optimum Bragg angle.
In constrast to the above-mentioned case with a single frequency wherein the beam incident angle to the acousto-optical element can be determined so as to maximize the primary diffraction intensity, the incident angle in case of plural frequencies is determined in the following manner.
FIG. 1 shows the change of the primary diffraction intensity as a function of incident angle .theta., in case the acousto-optical element receives two different frequencies.
The curves A and B respectively correspond to the frequencies .nu.A and .nu.B, having respective maximum points. Consequently, if the incident angle is selected corresponding to the maximum value of either one curve, the diffraction intensity for the other curve becomes inevitably lower. The two beams having such unbalanced diffraction intensities will provide different energy densities on the recording medium, thus giving rise to a difference in the density or in the spot diameter. In order to prevent such difference the incident angle is selected at .theta..sub.o, shown in FIG. 1, corresponding to the crossing point of said curves A and B, whereby the diffraction intensities corresponding to two frequencies are selected mutually equal, thus giving equal recording densities or spot diameters.
As the above-mentioned incident beam angle .theta..sub.o selected to obtain equal intensities in the diffracted beams does not correspond to the maximum diffraction intensity for each frequency, an eventual aberration of the actual incident angle from said selected angle .theta..sub.o will result in a difference among the diffraction intensities, leading to a difference in the recording density or in the spot diameter as explained in the foregoing.
An example of the scanning apparatus utilizing an acousto-optical element as the modulator and performing the scanning with plural independent beams is disclosed in Japanese Patent Laid-Open Sho53-101228, the optical system of which, from the laser to the modulator, is schematically there is shown in FIG. 2, wherein shown a laser 1, lenses 2, 3 for compressing the beam diameter, and an acousto-optical modulator 4. The beam compressing lenses 2 and 3 constitute an afocal lens system for reducing the diameter Bi of the incident laser beam from the laser 1 to an emergent beam diameter Bo. In response to a change of the laser beam angle from the laser by .DELTA..theta..sub.i, the beam emerging from the beam compressor and entering the acousto-optical modulator shows an angular change: EQU .DELTA..theta..sub.o =Bi/Bo.times.66.theta..sub.i ( 1)
Thus, in such optical system as explained in the foregoing, the angular change .DELTA..theta..sub.i in the beam emerging from the laser is amplified by Bi/Bo in the angular change .DELTA..theta..sub.o of the beam entering the acousto-optical modulator.